The invention relates to a device for altering the loop length, or loop height respectively, when weaving terry cloth with weaving machines in which the loop formation occurs via the movement of the fabric table.
In terry weaving machines the loops are produced by taking-up a so-called pile warp into the main warp. The take-up of the pile warp is performed with the weft insertion through the shed formed jointly by the pile warp and the main warp.
Terry weaving machines have the peculiarity that with a determined successive number of weft insertions, the weft thread is only partially beaten up, i.e. the weft thread is tied up in the shed, but not completely beaten up by the reed or comb right up to the fabric edge. With these wefts with partial beat-up, the fabric table moves from a starting position into a partial beat-up position further from the comb return point. After the prescribed number of partial beat-ups, a so-called complete beat-up occurs, in which the fabric table is normally brought into the starting position, i.e. into the position nearer the comb return point, the complete beat-up position. There all partially beaten-up weft threads are beaten-up at the fabric edge while the pile warp threads are push upwards and downwards, as a result of which the actual loops of the terry cloth. It is obvious that to define the complete or partial beat-up position, the position of the fabric edge, with respect to the comb return point, is crucial. The position of the fabric edge with respect to the comb return point is thus adjusted with the position of the fabric table.
The backward and forward movement of the fabric table is normally performed with a lever mechanism controlled by a cam.
EP 0,257,857 discloses a device for driving the fabric table in which the point of application of a driving rod on the lever arm can be altered so that the effective travel of the driving rod is shorter or longer.
This device has the disadvantage that to change the loop height a basic travel prescribed by the cam has to be superimposed. This means that with a variation from a desired loop height, in every terry cloth weaving cycle an adjustment device has to perform a correction to the travel which changes the partial beat-up position with respect to the complete beat-up position, i.e. the point of application of the driving rod has to be altered.
Therefore this device has the disadvantage that with each terry cloth weaving cycle the point of application of the driving rod has to be brought out of the starting position into a beat-up position and from there again into the starting position.
This results in a high degree of wear on highly-stressed parts of the actuating drive. However above all the demands on the precision of the device are even higher when the position of the complete and/or partial beat-up position of the fabric table has to be altered frequently.